fix(api): encode untyped datetime responses

tests/api/test_openapi_contract.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+from datetime import datetime, timezone
 from pathlib import Path
 from uuid import uuid4
 
@@ -281,3 +282,35 @@ def test_rest_runtime_wraps_handler_paginated_list() -> None:
         "limit": 2,
         "offset": 1,
     }
+
+
+def test_rest_runtime_json_encodes_untyped_datetime_response() -> None:
+    source_router = APIRouter()
+
+    @source_router.get("/simulation-result")
+    async def simulation_result():
+        return {
+            "result": [
+                {
+                    "time": datetime(2026, 7, 30, 4, tzinfo=timezone.utc),
+                    "id": "4277",
+                }
+            ]
+        }
+
+    app = FastAPI(redirect_slashes=False)
+    app.include_router(build_rest_router(source_router.routes), prefix="/api/v1")
+
+    response = TestClient(app, raise_server_exceptions=False).get(
+        "/api/v1/simulation-result"
+    )
+
+    assert response.status_code == 200
+    assert response.json() == {
+        "result": [
+            {
+                "time": "2026-07-30T04:00:00+00:00",
+                "id": "4277",
+            }
+        ]
+    }